The typical domestic water heater utilizes a central flue which is normally 3 to 4 inches in diameter depending, upon the burner input. The flue is normally 2 to 4 feet in length and connected at the bottom to an upward convex tank bottom or header. A circular burner is placed a few inches below the bottom. Gases from the burner are burned within the confines of the interior of the tank below the bottom, and flow up the central flue. A baffle is normally placed in the flue to maximize heat transfer between the hot gases and the sidewalls of the flue and thence into the water in the tank. The flue and bottom are uninsulated, because they are the prime heat exchange surfaces to conduct heat into the water. Efficiency of heat transfer from gases to surfaces is typically required to be at least 70%. A pilot is used to ignite the burner. Burnt gases from the pilot flow up the flue and retard heat loss from the exposed uninsulated surfaces of the flue and bottom. The baffling in the flue must be adjusted for the full firing rate of the burner when operating, to provide maximum efficiency with complete combustion of the flue gases. When the burner is not on, the pilot gases flowing up the flue are under baffled, because the pilot flow rate is only about 1 to 5% of that of the main burner. The exposed flue and heater bottom are cooled when the main burner is off, causing a loss of heat from the water inside the tank. This standby loss of heat must be replaced to maintain water temperature.
With the shortage of natural gas (methane) worldwide, and increasing costs, efforts have been taken in recent years to increase efficiency by adding insulation, and by minimizing flow of excess air going up the flue. In addition, there has been growing concern regarding air pollution caused by oxides of nitrogen (NOX) generated in the burner flames of a conventional water heater.
One Government agency has estimated that 12 tons of NOX issue daily from the 3,000,000 odd water heaters in the area served. The agency has called for a 50% reduction in such emissions; and is requiring that no water heater sold may have NOX emission of more than 40 nanograms per joule of heat generated. It is difficult and costly to attain such reduction with conventional burners, which normally operate with secondary air.